1. The Field of the Invention
The present invention relates to centrifuges and, more specifically, centrifuges wherein a magnetic force levitates and stabilizes at least a portion of the rotor system thereof.
2. The Relevant Technology
From DT 23 14 436 A1 a bearing and a damping system is known for the stabilization of a fast rotating centrifuge. The rotor is guided stable in the position without a mechanical connection between the rotor and the surrounding. The stabilizing forces are magnetic in nature with an additional damping system produced by magnetic, mechanical-hydraulic, and hydropneumatic forces interacting with the rotor.
In this prior invention the combination and interaction of different damping factors and their coupling to the rotor is difficult to realize and requires an extremely high technical expense.
U.S. Pat. No. 5,196,748 discloses a laminated structure of a superconducting magnetic bearing. The reference discloses at least two permanent magnets in opposite polarity which are sandwiched causing a high magnetic flux density gradient and hence a high stiffness. The magnetic structure is inside a superconducting hollow cylinder.
Furthermore, DE 42 32 869 A1 discloses a superconducting bearing and a corresponding procedure for operation. The bearing system comprises a shaft encircled by permanent magnetic rings and a superconducting body, each of the elements being disposed within a housing. The superconducting body is shaped so that it is cooled down as the shaft is upwardly moved to a position where the rotating shaft is in balance between the weight of the shaft and the magnetic forces applied to the shaft by the interaction between the magnets and the superconductor.
The primary disadvantage of the above system is the complicated in-operation procedure whereby the rotor with the permanent magnets are located close to the superconductor following which a field frozen cooling down procedure is used to reach the superconducting stage. This standby procedure is less reproducible with respect to the rotor position and is a tedious pre-operation procedure.
It is therefore an object of one embodiment of the present invention to design and construct a centrifuge and a method of use thereof so that during centrifugation the rotor is magnetically stabilized by one or two bearings and operates substantially free of mechanical contact and friction. It is a further object of one embodiment of the invention to use superconducting magnetic bearings to prevent or at least minimize the use and maintenance of the enormous electric and electronic circuit equipment of conventional active magnetic bearing designs.
In one embodiment, the material utilized for designing and constructing bearings is xe2x80x9chigh-temperature superconducting (HTS)xe2x80x9d, which has been shortened herein to xe2x80x9csuperconductingxe2x80x9d. Furthermore, the term xe2x80x9cpermanent magnetxe2x80x9d is frequently abbreviated to xe2x80x9cPMxe2x80x9d.
It is yet another object of one or more of the embodiments of the present invention to achieve high speed operation up to 180,000 RPM, substantial damping properties, low vibrating noise of the rotor shaft, and/or less driving power as well as safe operation with highly unbalanced rotor systems. A high bearing damping factor is desired at all operational frequencies. The damping should be independent of the actual speed.
The above objects are achieved in one embodiment with at least one permanent magnet configuration arranged at a distance in front of a passive superconducting magnet stator so as to produce an improved superconducting magnetic bearing.
Especially advantageous is the passive and self-stabilizing character of the bearing without any electronic control. As a result, the expenditure of the system is substantially reduced. The bearing allows a relative robust operation in experiment and processing.
In a preferred embodiment of the invention the permanent magnets are arranged so that the resulting magnetic flux distribution is directed almost normally to adjacent interacting surfaces of permanent magnets and superconductor.
In another preferred embodiment of the invention the rotor axis encircles an angel from 0 to 90 degrees relative to a horizontal plane.
In a highly preferred embodiment of the invention the rotor system has at least one ring or cylinder-shaped permanent magnet configuration and adjacent in radial direction a passive superconducting magnet stator.
In a first preferred embodiment of the invention the permanent magnet of the rotor is surrounded partly or totally by the superconducting stator element.
In a second preferred embodiment the permanent magnet configuration of the rotor concentrically surrounds the HTS magnet stator.
The rotor design advantageously consists of one PM configuration which at least in the axial direction is adjacent to at least one passive superconducting magnet stator. In another embodiment the permanent magnet configuration is attached on a shaft or a shaft-shaped part along the rotor axis. In the case of a multiple bearing design, at least one stator surrounds the shaft or part of the shaft.
In a further preferred embodiment of the centrifuge the permanent magnet configuration is fixed in a hollow cylinder-shaped shaft of the rotor or is designed as a hollow cylinder-shaped shaft. For more than one bearing, the hollow shaped shaft surrounds at least one stator.
The present invention also envisions a combination with a rotor arrangement with at least one PM configuration which is radially directed adjacent to a superconducting magnet stator and in addition is simultaneously adjacent to an axial magnet stator.
In a further preferred embodiment the passive superconducting magnet stator is connected with a cryogenic unit. The stator is at least partly shaped as a hollow cylinder. In one embodiment, the stator is a monolithic hollow cylinder of melt textured YBCO material with a critical temperature of 92 K equivalent xe2x88x92181xc2x0 C. The preferred stator material is polycrystalline melt textured material of the composition Y13-17Ba2Cu3O7-xcex4. The material shows a multi-domain magnetic structure with advantageous damping properties produced by the material grain boundaries for the magnetic bearing.
In an additional preferred embodiment the stator includes damping rings of copper, aluminum or alloys thereof. The damping devices can either be sandwiched and thermally connected between individual superconductors or fitted into the superconducting components.
The superconducting magnet stator is connected with a cryogenic engine inside the centrifugal housing in order to cool down the superconductor below its critical temperature. Alternatively, the superconductor can be cooled in an economical way by liquid Nitrogen to a temperature of 77 K or equivalent xe2x88x92196xc2x0 C.
In an alternative embodiment at least a portion of the superconducting magnet stator has a hollow cylinder or a ring shape with at least one lateral open segment or sector window.
The permanent magnet configuration in one embodiment has a ring or cylinder shape and is arranged coaxially with respect to the rotor axis.
According to another embodiment the permanent magnet configuration comprises as a ring or cylinder shaped PM. A plurality of co-axial rings and hollow cylinders are stacked directly or with another material therebetween in radial and/or axial direction.
One preferred embodiment of the centrifuge has a PM configuration comprising axial magnetized rings stacked or sandwiched in axial direction so that adjacent magnet surfaces are magnetized to the same polarity. As a result of the equation of continuous magnetic flux flow the desired high magnetic flux density expands radially of the rotatable member.
According to another embodiment of the centrifuge, at least two annular PM""s are coaxially fitted one in another in a common plane whereby the outer diameter of the smaller ring touches the inner diameter of the larger ring. The rings can be repulsive or attract each other by having adjacent PM faces either in the same or opposite axial polarity.
In a useful embodiment the permanent magnet configuration is mounted directly in the lower part of the rotor cup.
For the PM configuration at least one annular or cylindrical shaped magnet is typically needed.
In a further preferred embodiment the stator comprises at least one cylindrical finger made from aluminum or copper that is surrounded by and thermally connected to a hollow cylinder of superconducting material. The superconducting finger provides a stable magnetic interaction and levitation with the PM rings positioned inside the rotatable drive shaft.
In a first preferred embodiment of the novel centrifuge, the drive shaft with a driving motor system carries at least one pair of axially stacked PM""s. The magnets are arranged along the rotational axis in spaced apart groups. The PM groups are radially surrounded by superconducting magnet stator elements.
The drive shaft has a hollow shaped geometry because of weight and is fabricated from a material of high ultimate tensile strength. The material typically includes a metal or alloy comprised of aluminum, steel, titanium and/or carbon fiber composite whereby the ratio of tensile strength to density is typically in a range from about 106 Pa cm3/g to about 3xc3x97109 Pa cm3/g.
The drive shaft of this embodiment is connected with annular PM""s producing a radial flux distribution. The radial flux distribution is created either by axially magnetized PM rings stacked in groups in repulsive geometry with a high permeable, magnetic flux collecting and guiding material interposed therebetween or by radially magnetized annular PM""s.
In a further preferred embodiment the permanent magnet configuration is utilized simultaneously for the bearing function as well as the wheel driving motor function. Alternatively, the driving function can be due to a standard induction motor.
In a further preferred embodiment the superconducting magnet stator is modified so that copper rings are interposed between the superconducting cylinders. This positioning of the copper rings provides additional damping of the bearing by speed dependent eddy currents which increase the temperature of both the Cu rings and the adjacent HTS stator material slightly. The gap between the superconducting stator and the annular PM""s is typically between about 1 mm to about 5 mm.
The drive shaft has a mechanical emergency bearing at its lower part. The emergency bearing stabilizes the rotor part during the cool-down procedure of the superconductor or in the initial and final phase of the centrifugal operation at low speeds.
In a further preferred embodiment of the invention the housing is positioned at the upper end of the vertical rotor with at least two annular or cylindrical PM""s mounted on the shaft. Adjacent axial arranged passive superconducting magnets control the stabilizing magnetic forces in the axial direction. In contrast to the first embodiment, here the levitation forces are substantially axially directed.
Practically, the superconducting magnets are shaped as a ring or a hollow cylinder whereby the gap between rotor and stator can be closed by a thin isolation wall. In this configuration, the drive shaft together with permanent magnet rings rotate in a warm bore separated from the cold superconductors. The axial distance between each group of stacked PM""s correlates with the distance of the superconductors. The practical minimum distance between the stacked PM""s due to the magnetic interaction is about 5 mm. Increasing the distance between the double bearings along the drive shaft improves the tilt stiffness of the rotor.
Advantageously, the unbalance stability in a double-bearing centrifuge is improved when the gap between the PM and the superconductor is larger in the upper bearing near the centrifugal rotor compared to the gap distance in the lower bearing. This combination of different stiffnesses of the two bearings is able to control substantially large unbalanced masses relative to the state of the art of the centrifugal technique.
In a third preferred embodiment of the invention a rotor design includes a drive shaft having at one end thereof a hollow shaped cylinder. At least one permanent magnet ring or cylinder is disposed inside the hollow shaped cylinder. Centrally disposed within the permanent magnet is a passive superconducting magnet stator. The stator is concentrically arranged on a cooling finger and is connected to a cooling unit. The hollow shaft cylinder has at the outside a pole structure of an induction motor as a driving unit.
In a preferred fourth embodiment of the invention the rotor comprises a plurality of annular permanent magnets arranged in-plane as an integrated rotor part. Concentrically therewith and spaced apart therefrom, the superconductor magnet is positioned adjacent to the lower face of the PM configuration. The annular magnets are axially magnetized with equal polarities leading to a flux distribution parallel to the rotor axis and perpendicular to the neighboring functional faces. The magnet rings are in a repulsive position. Due to the magnet superconductor interaction and the pinning effect, the rotor is levitated at a certain distance from the bearing stator. The annular magnets have one degree of freedom to rotate contactless and almost frictionless. The driving system can also be contactless, e.g., due to an axial multi-pole induction motor.
Another preferred embodiment is a plane-like axial bearing with at least two concentrically and coaxially arranged PM rings fitted within each other equivalent to the geometry above. However, the magnetization of adjacent magnet ring surfaces has a different polarity and the fitted annular magnets are in an attracting position. Due to the high magnetic gradient in radial direction, the plane-like bearing is especially stiff for radial rotor displacement. The rotatable part is disposed above the plane-like cylindrical superconductor. Furthermore, by using xe2x80x9cfreezing,xe2x80x9d a quantity of the magnetic flux penetrated into the superconductor is trapped there by effective pinning centers in the material. The rotor system is stabilized by repulsive and attractive magnetic forces in an equilibrium state above the superconductor.
Finally, the present invention also includes a flat centrifuge design with a high load capacity and simultaneously high angular stiffness. One solution is the double-bearing system described above. Another solution utilizes a flat YBCO ring with a size of about 100 mm to about 400 mm in diameter with one or more PM rings inside or outside the superconductor. The annular rings carry the rotor in this single-bearing design. The resulting configuration is extremely stable and comprises a highly compact centrifuge construction.